1. Field of the Invention
The present invention relates to measuring the corrosion of a metal material. More particularly, it relates to a method of evaluating the corrosion resistance of a metal material, a method of designing an alloy of high corrosion resistance, a method of operating a plant, and a method of diagnosing the corroded state of a metal material. These methods are based on the analyzed results of a theoretical model for the corrosional damage mechanism of a plant material.
2. Description of the Related Art
The lifetime of a material in a plant directly affects that of the plant. It is therefore important, when designing a plant, to design the material and operation of the plant to reduce stress-corrosion cracking (SCC) which is especially undesirable among various types of corrosional damages of the material.
At present, techniques to be stated below are proposed as contrivances for countering SCC.
Disclosed in Japanese Patent Application Laid-open No. 333189/1993 is a technique wherein the remaining lifetime of a material which embrittles under irradiation with neutrons is measured from variations in the mechanical characteristic values of the material.
Japanese Patent Application Laid-open No. 223300/1992 discloses a method for prolonging the lifetime of a member in a newly built nuclear reactor or that of a member newly replaced in an existing nuclear reactor.
Japanese Patent Application Laid-open No. 179407/1993 discloses a method for providing a high-chromium stainless steel alloy whose SCC resistance is enhanced with respect to SCC in elevated-temperature water.
The prior-art techniques explained above are all considered as being phenomenal countermeasures to the SCC among various types of corrosional damage.
SCC is a phenomenon which arises when certain conditions of the environment, stress and material coincide. Regarding, for example, the structural material of a plant, SCC takes place initially in the case of coincidence of the conditions; (1) oxidation by oxidants (e.g., oxygen and hydrogen peroxide being those radiolytic products of water which exist in the cooling water of the plant), (2) the presence of a stress which acts on the material, and (3) the depletion or deficiency of chromium in the material. Accordingly, when any of the three elements is absent, SCC does not take place. The concentrations of oxygen etc. in the cooling water, for example, are environmental factors.
The intensity of the oxidizing power of a certain environment, in other words, the driving force thereof for the oxidation, can be indexed by a corrosion potential. As the oxidizing power of the environment is higher, the corrosion potential is higher. That is, as the corrosion potential is higher, SCC is more likely to arise. It is known, for example, that the SCC of the so-called "sensitized stainless steel", in which a chromium content at the grain boundary of the material has been lowered by carbonization of the chromium attributed to heat in a welding operation, arises at or near about -230 (mV vs. SHE (the potential of a standard hydrogen electrode)) in terms of the corrosion potential, and that it is conspicuously observed in the environment whose corrosion potential is higher.
At present, the fundamentals of the stress mode etc., the sensitized process, the basic theory, and so forth are elucidated up to a considerably high level, but phenomena at the boundary between the material and the cooling water environment (herein, especially the damaging phenomenon of the material) are not theoretically elucidated to satisfaction. It is the present situation that even the critical corrosion potential for the initiation of SCC, -230 (mV vs. SHE) as mentioned above cannot be theoretically interpreted.
In one of the prior-art techniques stated above, the critical corrosion potential for the initiation of SCC is assumed to be about -200 ((mV vs. SHE), and the oxidant concentration of the cooling water is controlled so as to render the corrosion potential thereof higher than the assumed value. However, various test conditions are involved in the experimental value (-200 mV), the significance of which is not clarified as explained above.
Accordingly, a question remains to whether or not the plant may actually be operated by setting the critical breakdown potential at -200 (mV).
If an environmental threshold value for the initiation of stress-corrosion cracking (herein, the critical corrosion potential for the initiation of SCC) can be theoretically supported, meritorious methodological approaches will be found for evaluating material, designing testing a material, for improving the environment of cooling water, etc. Heretofore, no technique has been proposed on the ground of a mechanism which uses common parameters in three regions; the corrosive environment of the material, a passive film to be formed on the material, and the metal matrix of the material.
There has not been any example in which the environmental threshold value in the breakdown of a passive film (especially, with the corrosion potential set as an index) is theoretically obtained using an electrochemical model for the semiconducting oxide film. Neither has been any example in which a method of operating a plant, a technique for designing an alloy or a technique for evaluating a corrosional damage is taught from such a viewpoint.
Incidentally, a theory for the dissolution of an ionic oxide film is disclosed in "Journal of Electrochemical Society", 113, 1067 (1966). The dissolution theory offers a theory according to which a potential gradient in an electric double layer predominates the dissolution rate of passive film, the predominance being the important point of a model for the breakdown of the passive film in the present invention. In the paper, however, the relationship of the potential gradient with a corrosional damage parameter is not stated at all, and material factors, corrosive environment factors and a stress which concern SCC are considered.
Information on the semiconducting film of an iron-chromium alloy disclosed in the Journal "Corrosion", 48, 229 (1992). It has been revealed that, when the chromium content of a semiconducting film increases, a flatband potential thereof lowers, whereupon the film exhibits more n-type characteristics. The paper, however, does not describe the relationship between the flatband potential and a corrosional damage parameter material factors, corrosive environment factors and stress which concern SCC are also not considered.